Gather-scatter operations

Except for the above gather-scatter operations, simulations are easily vectorizable as defined by the CRAY FORTRAN or the CYBER 200 FORTRAN. Typical vectors have 50 to 500 elements each. 

As we have seen on the CRAY the ability to gather data together is essential. Memory speed must be commensurate with floating point speed. When nearest neighbor tables are used fast scatter operations are also needed. The two essential random memory operations needed are ... 

A number of the most widely used collective communication operations provided by MPI are listed in Table A.3. The collective operations have been grouped into operations for data movement only (broadcast, scatter, and gather operations), operations that both move data and perform computation on data (reduce operations), and operations whose only function is to synchronize processes. In the one-to-all broadcast, MPl Bcast, data is sent from one process (the root) to all other processes, while in the all-to-all broadcast, MPI A11 gather, data is sent from every process to every other process (one-to-all and all-to-all broadcast operations are discussed in more detail in section 3.2). The one-to-all scatter operation, MPI Scatter, distributes data from the root process to all other processes (sending different data to different processes), and the all-to-one gather, MPI Gather, is the reverse operation, gathering data from all processes onto the root. 

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